1. Field of the Invention
The present invention relates to a method for manufacturing thermo-plastic sheets having an embossed pattern, and an apparatus therefor, to be used for manufacturing sheets having a reflective capability.
2. Description of Related Art
Recently, thermoplastic sheets having a reflective capability (plastic reflection panels), which are produced by performing a microprism process on the surface of thermo-plastic sheets, have been employed in the reflection panel field, the fashion field, and the architectural field.
In the United States, reflective sheets of the glass bead type and cube corner types have been approved as sheets having a reflective capability. In general, the glass bead reflective sheets are superior for short-distance visibility, and the cube corner reflective sheets are superior for long-distance visibility and luminance because of their reflective capability.
When manufacturing a reflective sheet of the cube corner type, an embossed pattern on an embossed pattern forming die must be transferred exactly to a thermo-plastic sheet.
It is especially important when processing a thermo-plastic sheet that it be pressed against an embossed pattern die at an adequate temperature and by the application of an adequate pressure, and that the application of the pressure be continued for a predetermined period of time.
A continuous pressing method (see Japanese Examined Patent Publication No. Sho 60-56103) and a belt method (see Japanese Examined Patent Publication No. Sho 5-17023) for manufacturing reflective sheets have been proposed as conventional methods that satisfy the above requirements.
With the continuous pressing method, an overlapping solid-state sheet material is continuously fed to a belt, and a plurality of pressing means having heating and cooling functions press embossed pattern molds against the sheet material to transfer patterns to the sheet material.
With the belt method, a belt bearing an embossed pattern mold and a thermoplastic sheet are gripped by a pair of rollers and pressed together to transfer the pattern to the sheet.
The production speed attained by the continuous pressing method, however, is slow, and the apparatus that is used is large and complicated.
Since for the belt method, the belt itself carries the embossed pattern, a problem arises concerning the durability of the belt that serves as the mold.
Although not presently being used for the manufacture of reflective sheets, the sheet manufacturing techniques described in Japanese Unexamined Patent Publication No. Hei 6-55613, Japanese Unexamined Utility Model Publication No. Hei 3-6919, and Japanese Unexamined Utility Model Publication No. Hei 1-83092 could be applied for the belt method for the manufacture of reflective sheets from a sheet resin that is melted and extruded through a die by an extruder.
However, were the technique described in Japanese Unexamined Patent Publication No. Hei 6-55613 used, the sheet would be insufficiently transparent when a nucleus-generating agent did not contain any additives. Also, were the technique in Japanese Utility Model Publication No. Hei 3-6919 used, if the molding speed was increased, the surface glossiness on the belt side would be degraded because the face pressing interval would be too short. Any technique in Japanese Unexamined Utility Model Publication No. Hei 1-83092 was used, even though an elastic member was employed, as this would be a belt that was coated with rubber, a problem would arise concerning its durability.